The invention relates to a new type of use for a water-soluble glass to protect glassware from corrosion during washing and/or rinsing cycles of a dishwashing machine, compositions intended for use in a dishwashing machine for the aforesaid purpose and a method of inhibiting the corrosion of glassware during washing and/or rinsing cycles of a dishwashing machine.
The problem of glassware corroding during washing and/or rinsing cycles of a dishwashing machine has long been known. Current opinion is that the problem of corrosion in glassware is the result of two separate phenomena. On the one hand, the corrosion is clearly due to minerals escaping from the glass composition accompanied by hydrolysis of the silicate network. On the other hand, silicate material is released from the glass. After several washes in a dishwashing machine, both phenomena can cause damage to glassware such as cloudiness, scratches, streaks and similar.
Silicate compounds are known to be effective in preventing minerals from being released from the glass composition but on the other hand can tend to increase the separation of silicate material at the surface of the glass.
Various proposals have been put forward as a means of dealing with the problems described above.
One approach is to use zinc, either in metallic form (U.S. Pat. No. 3,677,820) or in the form of zinc compounds. The use of soluble zinc salts as a means of preventing the corrosion of glassware in dishwasher detergents is described in U.S. Pat. No. 3,255,117, for example.
Because of a number of disadvantages inherent in using soluble zinc salts (in particular the formation of a precipitate of insoluble zinc salts with other ions in the washing or rinsing water), European patent applications EP 0 383 480, EP 0 383 482 and EP 0 387 997 propose the use of insoluble zinc compounds as a means of inhibiting the corrosion of glassware in automatic dishwashing machines. Specifically, the insoluble zinc salts proposed are zinc silicate, zinc carbonate, zinc oxide, basic zinc carbonate (approximately: Zn2(OH)2CO3), zinc hydroxide, zinc oxalate, zinc monophosphate (Zn3(PO4)2) and zinc pyrophosphate (Zn2(P2O7)). When using zinc salts of this type in granular cleansing compounds, the insoluble zinc compound is specified as having a maximum particle size of less than 1.7 mm (EP 0 383 482), whilst a mean particle size of less than 250 xcexcm is specified for the insoluble zinc compound used in a liquid dishwasher compound (EP 0 383 480 and EP 0 387 997).
The disadvantage of this prior art essentially resides in the fact that, because the zinc compounds are not readily soluble or are insoluble, it is difficult to ensure that a sufficient quantity of active agent will always be present in the washing or rinsing liquid to protect glassware from corrosion. Furthermore, in view of the high specific density of the insoluble zinc compounds listed, problems of separation arise with powdered mixtures or settlement in the case of liquid mixtures.
Finally, all of the known compositions are intended to be active during only one specific stage of the washing cycle, i.e. if admixed with a granular detergent composition during the washing cycle or if admixed with a liquid rinsing composition with the rinsing cycle. None of the known compositions has the capacity to become and remain active starting from the pre-wash cycle and/or one of the intermediate rinsing cycles onwards.
The underlying objective of the present invention is to resolve at least one, but preferably all of the existing problems outlined above.
This objective is achieved by the invention due to the use of a water-soluble glass to protect glassware from corrosion, the water-soluble glass being made from at least one compound which releases an active anti-corrosion agent during the washing and/or rinsing cycles of a dishwashing machine, each of these compounds accounting for no more than 85 molar % of the glass and the solubility of the water-soluble glass being defined by a mass loss of at least 0.5 mg under the conditions specified in DIN ISO 719.
By preference, the solubility of the water-soluble glass is such that the mass loss under the specified conditions is at least 10 mg, more preferably at least 50 mg.
The invention additionally proposes that the compound(s). which release(s) an active agent to protect glassware from corrosion during the washing and/or rinsing cycles of a dishwashing machine should be from the group consisting of the oxides of zinc, aluminium, tin, magnesium, calcium, strontium, titanium, zirconium, manganese and/or lanthanum and/or precursors thereof.
In one particular embodiment, the invention proposes that at least one of the compounds used should be zinc oxide and/or a precursor thereof.
By preference, at least one of the glass-forming components of the water-soluble glass is phosphorus pentoxide and/or a precursor thereof.
Also particularly preferred as a glass component(s) is (are) one or more alkali metal oxides and/or precursors thereof.
In one especially preferred embodiment, the invention proposes using the water-soluble glass in the form of a tablet, the tablet preferably being made by casting or drawing the water-soluble glass.
Alternatively, the water-soluble glass is used in crushed form, either in the form of glass platelets or in around form.
In ground form, the mean particle size is preferably at most 500 xcexcm.
The invention additionally relates to a composition for use in a dishwashing machine, containing an active quantity of a water-soluble glass in crushed form to protect glassware from corrosion, the water-soluble glass being made from at least one compound which releases an active agent to protect glassware from corrosion during washing and/or rinsing cycles of a dishwashing machine, each of these compounds accounting for at most 85 molar % of the glass and the solubility of the water-soluble glass being defined by a mass loss of at least 0.5 mg under the conditions specified in DIN ISO 719.
The water-soluble glass is preferably used either in the form of thin glass platelets or in ground form, most preferably with a mean particle size of at most 500 xcexcm.
The composition proposed by the invention contains the water-soluble glass in a quantity of from 0.1 to 10.0% by weight, more preferably from 0.5 to 5.0% by weight.
In an alternative embodiment, the invention proposes a composition for use in a dishwashing machine, characterized in that it contains an active quantity of a water-soluble glass tablet form to protect glassware from corrosion, the water-soluble glass being made from at least one compound which releases an active agent to protect glassware from corrosion during washing and/or rinsing cycles of a dishwashing machine, each of these compounds accounting for at most 85 molar % of the glass and the solubility of the water-soluble glass being defined by a mass loss of at least 0.5 mg under the conditions specified in DIN ISO 719.
The tablet proposed by the invention is preferably made by casting or drawing the water-soluble glass.
With both the composition in which the water-soluble glass is present in crushed form and in the embodiment where the water-soluble glass is provided as a tablet, the solubility of the water-soluble glass is such that the mass loss under the specified conditions is preferably at least 10 mg, more preferably at least 50 mg.
In both cases, the compound(s) which release(s) an active agent to protect glassware from corrosion during washing and/or rinsing cycles of a dishwashing machine is (are) from the group consisting of the oxides of zinc, aluminium, tin, magnesium, calcium, strontium, titanium, zirconium, manganese and/or lanthanum and/or precursors thereof.
In one particular embodiment, the invention proposes that at least one of the compounds should be zinc oxide and/or a precursor thereof.
Preferably, at least one of the glass-forming components of the water-soluble glass is phosphorus pentoxide and/or a precursor thereof.
One or more alkali metal oxides and/or precursors thereof may also be used as glass component(s).
The invention additionally relates to a method of inhibiting the corrosion of glassware in washing and/or rinsing cycles of a dishwashing machine by bringing the glassware into contact with washing and/or rinsing water containing an active quantity of a composition containing the water-soluble glass in ground form.
Alternatively, a composition as proposed by the invention is prepared in the form of a tablet, placed in the interior of the dishwasher at a point which is accessible to the washing and/or rinsing water.
Before giving a detailed explanation of the features and advantages of the present invention, it should be pointed out that for the purpose of the present invention, the concept xe2x80x9cglassxe2x80x9d should be construed as meaning not only glass in the narrower sense, i.e. an amorphous, non-crystalline, transparent substance produced by hardening a melt, but also that produced by hardening a melt which has crystalline inclusions in an amorphous glass matrix.
The present invention solves at least one and, in preferred embodiments, all the problems inherent in the prior art described above in a surprisingly simple manner.
By using a water-soluble glass as a xe2x80x9cbasexe2x80x9d for one or more reagents to protect glassware from corrosion in a dishwasher, the problems caused by the extensive formation of precipitate from insoluble salts which result in an undesirable deposit, such as occurs with the prior art in particular due to the use of soluble zinc salts for this purpose, are avoided. Instead, the active agents, such as zinc ions, for example, are released from the water-soluble glass proposed by the invention into the washing or rinsing water in a delayed release pattern so that there is always only a small concentration of active agents present. In this concentration, although the agents are effective in protecting glassware against corrosion, they are not present in a high enough concentration to lead to the undesirable formation of insoluble salts to any significant degree. The disadvantages of using insoluble zinc compounds described above are therefore also avoided by using the water-soluble glass proposed by the invention.
By using the water-soluble glass proposed by the invention in ground form as an additive to standard dishwasher products, which are generally made as powders or liquids, the problem of separation described above is also resolved since the use of a water-soluble glass offers greater flexibility in terms of adjusting the specific density to requirements.
If the water-soluble glass proposed by the invention is provided in tablet form and placed in the interior of the dishwasher at a point which is accessible to the washing and/or rinsing water, e.g. in the cutlery basket, it will also provide, for the first time ever, active protection against corrosion throughout all washing and rinsing cycles, i.e. from the pre-rinse cycle through to the cleaning cycle and then the intermediate rinse cycles through to the final rinse cycle and, what is more, will last for several cycles. As a result, not only is glassware thoroughly protected against corrosion in the dishwasher, handling is made significantly easier and more convenient for the consumer.
The glasses proposed by the invention may be produced by melting mixtures of the oxide components or precursors thereof for a sufficient time to obtain a homogeneous melt, which is then cooled until it solidifies.
The tablets may be made using various methods, for example by casting (e.g. in a graphite mould), drawing, pressing or blowing.
If the glass proposed by the invention is used in crushed form, it may be ground in a grinder specifically suited to this purpose, for example, and an appropriate fraction of particles separated out, e.g. having an average particle size of at most 500 xcexcm. However, particulate glass may also be produced by other methods, e.g. by breaking down thin glass discs, resulting in disc-shaped, thin platelets which may be up to several millimeters in diameter. Both embodiments, i.e. spherical particles with a specific maximum particle size and thin platelets, will reliably ensure thorough mixing with the dishwasher detergent and prevent separation during storage and transportation.
The solubility of the glass used is defined in accordance with DIN ISO 719. This test method tests glass as a material and is conducted on glass fines. Grains of a size of between 300 and 500 xcexcm are extracted from 2 g of fines over a period of 60 min with water conforming to Quality 2 at 98xc2x0 C. Further details of how the test is conducted can be found in the relevant standard. However, by contrast with the aforementioned standard, the degree of solubility is determined not by titration with acid but by a standard gravimetric definition of the mass loss.